4 files changed, 78 insertions, 22 deletions

diff --git a/src/corelib/io/qprocess.cpp b/src/corelib/io/qprocess.cpp
index 8e8664fda4..483c99ed5f 100644
--- a/src/corelib/io/qprocess.cpp
+++ b/src/corelib/io/qprocess.cpp
@@ -1529,7 +1529,7 @@ QProcess::CreateProcessArgumentModifier QProcess::createProcessArgumentsModifier
     \note This function is available only on the Windows platform and requires
     C++11.
 
-    \sa QProcess::CreateProcessArgumentModifier
+    \sa QProcess::CreateProcessArgumentModifier, setChildProcessModifier()
 */
 void QProcess::setCreateProcessArgumentsModifier(CreateProcessArgumentModifier modifier)
 {
@@ -1539,6 +1539,59 @@ void QProcess::setCreateProcessArgumentsModifier(CreateProcessArgumentModifier m
 
 #endif
 
+#if defined(Q_OS_UNIX) || defined(Q_QDOC)
+/*!
+    \since 6.0
+
+    Returns the modifier function previously set by calling
+    setChildProcessModifier().
+
+    \note This function is only available on Unix platforms.
+
+    \sa setChildProcessModifier()
+*/
+std::function<void(void)> QProcess::childProcessModifier() const
+{
+    Q_D(const QProcess);
+    return d->childProcessModifier;
+}
+
+/*!
+    \since 6.0
+
+    Sets the \a modifier function for the child process, for Unix systems
+    (including \macos; for Windows, see setCreateProcessArgumentsModifier()).
+    The function contained by the \a modifier argument will be invoked in the
+    child process after \c{fork()} is completed and QProcess has set up the
+    standard file descriptors for the child process, but before \c{execve()},
+    inside start(). The modifier is useful to change certain properties of the
+    child process, such as setting up additional file descriptors or closing
+    others, changing the nice level, disconnecting from the controlling TTY,
+    etc.
+
+    The following shows an example of setting up a child process to run without
+    privileges:
+
+    \snippet code/src_corelib_io_qprocess.cpp 4
+
+    If the modifier function needs to exit the process, remember to use
+    \c{_exit()}, not \c{exit()}.
+
+    \note In multithreaded applications, this function must be careful not to
+    call any functions that may lock mutexes that may have been in use in
+    other threads (in general, using only functions defined by POSIX as
+    "async-signal-safe" is advised). Most of the Qt API is unsafe inside this
+    callback, including qDebug(), and may lead to deadlocks.
+
+    \sa childProcessModifier()
+*/
+void QProcess::setChildProcessModifier(const std::function<void(void)> &modifier)
+{
+    Q_D(QProcess);
+    d->childProcessModifier = modifier;
+}
+#endif
+
 /*!
     If QProcess has been assigned a working directory, this function returns
     the working directory that the QProcess will enter before the program has
@@ -1843,25 +1896,16 @@ void QProcess::setProcessState(ProcessState state)
     emit stateChanged(state, QPrivateSignal());
 }
 
+#if QT_VERSION < QT_VERSION_CHECK(7,0,0)
 /*!
-  This function is called in the child process context just before the
-    program is executed on Unix or \macos (i.e., after \c fork(), but before
-    \c execve()). Reimplement this function to do last minute initialization
-    of the child process. Example:
-
-    \snippet code/src_corelib_io_qprocess.cpp 4
-
-    You cannot exit the process (by calling exit(), for instance) from
-    this function. If you need to stop the program before it starts
-    execution, your workaround is to emit finished() and then call
-    exit().
-
-    \warning This function is called by QProcess on Unix and \macos
-    only. On Windows and QNX, it is not called.
+    \internal
 */
-void QProcess::setupChildProcess()
+auto QProcess::setupChildProcess() -> Use_setChildProcessModifier_Instead
 {
+    Q_UNREACHABLE();
+    return {};
 }
+#endif
 
 /*! \reimp
 */
diff --git a/src/corelib/io/qprocess.h b/src/corelib/io/qprocess.h
index 9e497c3871..77980e012c 100644
--- a/src/corelib/io/qprocess.h
+++ b/src/corelib/io/qprocess.h
@@ -205,6 +205,10 @@ public:
     CreateProcessArgumentModifier createProcessArgumentsModifier() const;
     void setCreateProcessArgumentsModifier(CreateProcessArgumentModifier modifier);
 #endif // Q_OS_WIN || Q_CLANG_QDOC
+#if defined(Q_OS_UNIX) || defined(Q_CLANG_QDOC)
+    std::function<void(void)> childProcessModifier() const;
+    void setChildProcessModifier(const std::function<void(void)> &modifier);
+#endif
 
     QString workingDirectory() const;
     void setWorkingDirectory(const QString &dir);
@@ -263,8 +267,6 @@ Q_SIGNALS:
 protected:
     void setProcessState(ProcessState state);
 
-    virtual void setupChildProcess();
-
     // QIODevice
     qint64 readData(char *data, qint64 maxlen) override;
     qint64 writeData(const char *data, qint64 len) override;
@@ -273,6 +275,15 @@ private:
     Q_DECLARE_PRIVATE(QProcess)
     Q_DISABLE_COPY(QProcess)
 
+#if QT_VERSION < QT_VERSION_CHECK(7,0,0)
+    // ### Qt7: Remove this struct and the virtual function; they're here only
+    // to cause build errors in Qt 5 code that wasn't updated to Qt 6's
+    // setChildProcessModifier()
+    struct Use_setChildProcessModifier_Instead {};
+    QT_DEPRECATED_X("Use setChildProcessModifier() instead")
+    virtual Use_setChildProcessModifier_Instead setupChildProcess();
+#endif
+
     Q_PRIVATE_SLOT(d_func(), bool _q_canReadStandardOutput())
     Q_PRIVATE_SLOT(d_func(), bool _q_canReadStandardError())
     Q_PRIVATE_SLOT(d_func(), bool _q_canWrite())
diff --git a/src/corelib/io/qprocess_p.h b/src/corelib/io/qprocess_p.h
index 2587530c09..7cb70cc95c 100644
--- a/src/corelib/io/qprocess_p.h
+++ b/src/corelib/io/qprocess_p.h
@@ -322,6 +322,8 @@ public:
 #if defined(Q_OS_WIN)
     QString nativeArguments;
     QProcess::CreateProcessArgumentModifier modifyCreateProcessArgs;
+#else
+    std::function<void(void)> childProcessModifier;
 #endif
     QProcessEnvironment environment;
 
diff --git a/src/corelib/io/qprocess_unix.cpp b/src/corelib/io/qprocess_unix.cpp
index 50390e57f5..f14e78f94e 100644
--- a/src/corelib/io/qprocess_unix.cpp
+++ b/src/corelib/io/qprocess_unix.cpp
@@ -459,7 +459,7 @@ void QProcessPrivate::startProcess()
     // safe with vfork semantics: suspend the parent execution until the child
     // either execve()s or _exit()s.
     int ffdflags = FFD_CLOEXEC;
-    if (typeid(*q) != typeid(QProcess))
+    if (childProcessModifier)
         ffdflags |= FFD_USE_FORK;
     pid_t childPid;
     forkfd = ::forkfd(ffdflags , &childPid);
@@ -544,7 +544,6 @@ void QProcessPrivate::execChild(const char *workingDir, char **argv, char **envp
 {
     ::signal(SIGPIPE, SIG_DFL);         // reset the signal that we ignored
 
-    Q_Q(QProcess);
     ChildError error = { 0, {} };       // force zeroing of function[8]
 
     // copy the stdin socket if asked to (without closing on exec)
@@ -574,8 +573,8 @@ void QProcessPrivate::execChild(const char *workingDir, char **argv, char **envp
         goto report_errno;
     }
 
-    // this is a virtual call, and it base behavior is to do nothing.
-    q->setupChildProcess();
+    if (childProcessModifier)
+        childProcessModifier();
 
     // execute the process
     if (!envp) {